1. Field of the Invention
The invention pertains to the field of chain tensioners. More particularly, the invention pertains to a tensioner that has damping.
2. Description of Related Art
A tensioning device, is used as a control device for a power transmission chain, or similar power transmission devices, as the chain travels between a plurality of sprockets. In this device, the chain transmits power from a driving shaft to a driven shaft, so that part of the chain is slack and part of the chain is tight. Generally, it is important to impart and maintain a certain degree of tension in the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain. Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative.
However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension. For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. Reverse rotation of an engine, occurring for example during the stopping of the engine or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain.
Blade tensioners are tensioners that are commonly used to control a chain or belt where load fluctuations are not so severe as to over flex the spring or springs. A ratchet with backlash is often added to tensioners to limit the effective backward or untensioned travel of a tensioning device.
FIG. 1 shows a prior art chain driving system having a blade tensioner and a guide. A closed loop chain encircles driving sprocket 12 and driven sprocket 10. Each sprocket 10, 12 accelerates and decelerates independently while maintaining forward motion. A fixed guide 14 is attached to a bracket 7 on the tight strand of the chain 8. Opposite the guide 14 on the slack strand of the chain is a tensioner 16, which is at least semi-rigidly fixed to the bracket 7 and biased towards the tight strand of the chain. Bolts 18 fasten the bracket containing the tensioner 16 and the guide 14 to the engine block (not shown).
When the driving sprocket 12 accelerates or the driven sprocket 10 decelerates, an energy wave or high local load is created in the tight strand of the chain, which travels from the sprocket that changed in velocity toward the other sprocket. The chain 8 attempts to span the distance between the link of the chain in contact with the initiating sprocket to the other sprocket in the shortest possible distance, a straight line. The energy moves through the links on the free strand of the chain until it comes to the end of the guide 14, which absorbs the energy. As a result of the constant absorption of the high local load, the ends of the guide 14 sustain significant wear. The energy wave would be present and show the same characteristics if it was to originate on the slack strand of the chain, although the wave energy may not occur at all depending on how well the tensioner keeps the slack out of the chain and how much damping the tensioner has.
One solution to reduce the load that results in the prior art chain driving system is to add damping to the tensioner. One problem associated with adding the required damping to a tensioner is that if the tensioner is softened significantly, it may result in a loss of chain control, since tensioners are specifically designed to apply just enough force to provide adequate chain control. Other solutions are shown in U.S. Pat. Nos. 5,797,818, and 6,322,470, as well as US Publication Nos. 2002/0045503 and 2005/0085322.
U.S. Pat. No. 5,797,818 discloses a chain assembly with reduced noise and vibration on the slack side of the chain that includes a blade shoe, a spring, and a resilient damping device. The shoe is pivotally mounted to the tensioner bracket. The spring is mounted to the shoe and biases the shoe towards the chain. The resilient damping device is mounted between the tensioner bracket and the spring to aid in controlling the vibration that occurs during valve events.
U.S. Pat. No. 6,322,470 discloses a tensioner that includes a pair of pivoting arms used to simultaneously tension two separate strands of the same chain. A lever with fixed pins is located between the two strands. The arms extend outside the strands of the chain and carry shoes positioned to contact the outside portion of the chain which are pivotally mounted to the fixed pins. Rotation of the lever causes the fixed pins to move laterally and the arms draw inward and tension is imparted to the separate chain strands at the same time. In other words, a high load on one side of the chain tries to transfer to the other side of the chain but the load eventually evens out.
US Publication No. 2002/0045503 discloses a chain tensioner that has a blade and spring assembly. The blade has a first end and a second end. The first end defines a hub. The hub is connected to the central portion of the blade shoe and a first end wall and defines a bore that receives a single pin that pivotably connects the blade to a mounting bracket.
FIG. 2 shows the pivoting chain guide and tensioner assembly of US Publication No. 2005/0085322. It consists of a chain guide 124 and a tensioner each in contact with one of the two strands of a closed loop chain 108 in a dual sprocket chain system, the chain 108 being routed around a drive sprocket 112 and a driven sprocket 110. The tensioner and chain guide are both connected to a bracket plate 122 which pivots about a single mounting point 120.
As shown in FIG. 3, a conventional chain tensioning system consists of two independently functional blade spring tensioners 306 and 306a that are located on each of the two strands of chain 300. These separately functional tensioners each include a bracket body 307, 307a, which each contain chain guides 308 and 308a. Within each bracket body 307 and 307a are at least one blade spring, identified as 510 (see FIG. 4) to urge chain guides 308 and 308a into forcible contact with the strands of chain 300 to which each mechanical spring tensioner 306 and 306a are adjacent. In this conventional system, each mechanical spring tensioner operates independently of the other. Each mechanical spring tensioner is securely mounted to the engine block, not shown, by securing means 312, which may be, for example bolts or press fit pins. The only motion provided to adjust the tension between the slack and tight strand of chain 300 comes from the blade springs.